Hydrogen is one of the most important industrial gases. In 1981 nearly 4.times.10.sup.12 scf of hydrogen were produced and consumed in the United States. The bulk of this hydrogen is used in ammonia synthesis and in the petroleum refining industry, both in hydrocracking and hydrodesulfurization, and as a refinery fuel. The remainder is used in methanol synthesis, the aerospace industry and other specialized applications.
Hydrogen is produced from coal, methane, or oil, and must be separated from carbon dioxide before use, a factor which contributes significantly to the cost of the gas. Also, a number of industries using hydrogen produce bleed streams and off-gases containing significant quantities of hydrogen which are too impure to recycle without treatment. Recovery and re-use of hydrogen is frequently desirable in these cases. For example, in ammonia synthesis plants, recycling of hydrogen from purge gases containing hydrogen, argon, and methane can improve operating efficiency up to 5%.
Conventional techniques used to purify include cryogenic units and various absorption and adsorption processes. For hydrogen recovery in ammonia plants, cryogenic units have traditionally been the principal method used. Where very high purity hydrogen is needed, pressure swing adsorption, which can produce hydrogen that is 99.999% pure in a single pass, has been the preferred method.
As an alternative to this conventional technology, in recent years synthetic semipermeable membrane processes have been developed which can be used for hydrogen separation, and in fact, more than 60 membrane-based hydrogen generating plants are now in operation worldwide. Most of these plants separate hydrogen from nitrogen, argon, or methane. The more common problem, that of separating hydrogen from carbon dioxide, cannot be tackled economically by existing membranes, which are insufficiently selective. The polymer membranes used in existing plants are asymmetric in structure, consisting of an extremely thin permselective layer, 1 micron or less in thickness, supported on a microporous, non-selective support layer. Because the permselective layer is thin, the permeate fluxes are relatively high, and compact plants are possible. Apart from a gas compressor, membrane-based units have no moving parts, and are adaptable to large- or small-scale operations. Membrane processes are also relatively energy efficient when compared with other technologies. However, present polymer membranes are unable to produce hydrogen with a purity of 99% from feed streams containing 80% or less hydrogen in a single step. They also cannot operate in circumstances where high temperatures, in excess of 100.degree. or 150.degree. C. are involved. If hydrogen-separating membranes are to be widely applied to low-pressure, low-hydrogen-concentration feed streams, or to the separation of hydrogen from carbon dioxide, a major increase in membrane performance is needed. For applications where the separation is to be performed at high temperatures, such as for example in a fuel cell, there is also a need for high-flux, temperature-resistance membranes.
Palladium and palladium-alloy membranes are highly selective and operate at high temperatures. Because hydrogen has the unique property of dissolving in the metal membrane, a defect-free palladium membrane can produce completely pure hydrogen in one pass. These membranes were extensively studied during the 1950's and 60's, and this work led to the installation by Union Carbide of a plant to seperate hydrogen from a refining off-gas stream containing methane, ethane, carbon monoxide and hydrogen disulfide. The plant was able to produce 99.9% or better pure hydrogen in a single pass through the membrane. The plant operated with 25-micron-thick membranes, at a temperature of 370.degree. C., and a feed pressure of 450 psi. The flux through the membranes was very low, however, and the process never became widely used. The low-flux problem could have been overcome by making much thinner membranes, because a decrease in the membrane thickness to 0.5 micron or less would increase the flux fifty-fold, to a level comparable with that of current composite polymer membranes. At the time however, ways to make ultrathin metal structures were not known, and the idea was dropped.
Nevertheless, there is an extensive patent literature which reflects the interest at the time in palladium membranes, U.S. Pat. Nos. 2,848,620 (1958) and 2,958,391 (1960) describe hydrogen separation using a thin film of palladium on a support of sintered metal particles. U.S. Pat. No. 3,232,026 (1966) teaches the use of palladium foils coated with palladium black for separating hydrogen from gas mixtures. U.S. Pat. Nos. 3,413,777 (1968) describe a "diffusion element" of a thin film of palladium in a vitreous glaze on a ceramic support. U.S. Pat. No. 3,630,690 (1971) discloses a hydrogen exhaust pump for use in hydrocracking containing a palladium substrate layer. U.S. Pat. No. 3,678,654 (1972) teaches a pervaporation-type process for recovery of dissolved hydrogen from water using a palladium-silver alloy tubes coated with palladium black. U.S. Pat. No. 4,254,086 (1981) describes a ceramic substrate coated with thin films of palladium and used in the thermal dissociation of water. In general, the palladium layers in each of these inventions are relatively thick, typically of the order 1 mil. There are also many patents covering palladium alloys useful for gas separation. For example, U.S. Pat. No. 2,773,561 (1956) claims a gas separation process using palladium-silver alloys; U.S. Pat. No. 3,155,467 (1964) describes alloy "walls" containing gold, ruthenium, or platinum; U.S. Pat. No. 3,172,742 (1965) discloses alloys suitable for hydrogen diffusion incorporating a percentage of uranium; U.S. Pat. Nos. 3,238,700 (1966) covers palladium/ruthenium alloys; 3,350,845 (1967) covers palladium/gold alloys; U.S. Pat. No. 3,713,270 (1973) describes the advantages of palladium/cerium or palladium/yttrium membranes, and so on. Again the thickness of the metal films in these patents is typically a few mils. It is also known that metals other than palladium may exhibit solution and diffusion of atomic hydrogen. For example, U.S. Pat. No. 2,958,391 discusses hydrogen permeation through films made from other metals in Group 8 of the Periodic Table such as iron, nickel, copper and platinum. Metals from Groups 4B, 5 B and 6B of the Periodic Table such as zirconium, titanium, niobium, vanadium, tantalum and molybdenum are also known to permeate hydrogen. The ability of other gases to permeate metal films is also known, for example, from a paper by Mercea et al. entitled "Permeation of Gases through Metallized Polymer Membranes", (Journal of Membrane Science, Vol. 24, 1985).
However, because of their very low fluxes, their need to operate at high temperatures and high feed pressures, and their expense, metal membranes have been generally considered inferior to polymer membranes, and have found no large-scale industrial application. With the development of modern vacuum deposition techniques, however, a method is now available to make much thinner metal films than was previously possible. U.S. Pat. No. 4,132,668 describes a method of making a hydrogen-permeable palladium membrane catalyst. The structure comprises a substrate of a porous metal sheet, such as stainless steel, an intermediate silicone rubber layer, and a catalytically active layer of palladium or palladium alloy, which is 0.1 micron or less thick, deposited on the polymer layer. U.S. Pat. No. 3,468,781 describes a membrane for use in a polarographic cell. The membrane comprises a layer of palladium or palladium alloy, typically 200 .ANG. or less thick, on a non-porous polystyrene film or similar support. Because of the thickness of the polymer layer employed in each of the above cases, however, the permeate flux is still far too low to be economically competitive. Thus, despite the prior teachings of the art, there remains a need for high-flux, high-selectively, membrane-based gas separation systems.